Undesirable high-voltage sparkovers can occur in cathode ray tubes of monitors; such sparkovers are caused by increased field intensities between the focusing unit and the anode, i.e., the inner coating of the tube or by impurities in the tube. Depending on the type of tube and the level of anodic voltage, very high currents may flow within very short times, permanently damaging the monitor electronics. Furthermore, these currents generate magnetic fields that induce voltages in the electronic components close to the tube. This creates the hazard of permanent damage to these components as well.
Circuit design measures can largely reduce the effect of a high-voltage sparkover. In this case, the amplitude of the current during sparkover is usually reduced using series resistors. These protective measures, however, limit the video band width of the monitor, with the result that high image resolutions, e.g., resolutions of up to 4000 lines for band widths of up to 600 MHz, are difficult to implement.
One cathode ray tube is described in German Utility Model 89 05 156. In order to reduce the damaging effects from high-voltage sparkovers, this German Utility Model describes a strip made of electrically conductive material and connected to ground potential be applied is parallel with the inner coating to the outer coating of the cathode ray tube. In the event of a high-voltage sparkover, which usually occurs between the anode and the focusing unit of the tube, the current flows in the tube via the focusing unit, the screen grid, the control grid, the cathode, and the cathode heater to ground, these elements, particularly the cathode and the outer protective circuit of the monitor electronics, being subject to damage.